Time ratio control and inverter power circuits



Dec. 30, 1969 R. E. MORGAN 3,487,

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TIME RATIO CONTROL AND INVERTER POWER CIRCUITS Original Filed Dec. 26, 1963 13 Sheets-Sheet 15 E/Voqfan K AiZf i ey [n veflor' Faymavd by i6 United States Patent 3,487,234 TIME RATIO CONTROL AND INVERTER POWER CIRCUITS Raymond Evan Morgan, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Original application Dec. 26, 1963, Ser. No. 334,690, now Patent No. 3,360,712, dated Dec. 26, 1967. Divided and this application Apr. 12, 1967, Ser. No. 630,346

Int. Cl. HOSE: 17/78 U.S. Cl. 307252 21 Claims ABSTRACT OF THE DISCLOSURE A family of time ratio control D-C power circuits comprises a load current carrying SCR, triac, diac, or dv/dt fired SCRwhich is turned on and commutated off at desired intervals to supply power to the load, which is connected in series with a filter inductance and in parallel with a coasting diode for inductive loads. The commutation circuit comprises a cornmutating capacitor in series with a parallel-connected linear inductance and saturable reactor and with an auxiliary SCR-diode combination or triac. The capacitor-linear inductance are tuned to series resonance at a frequency substantially lower than the cornmutating frequency, while the capacitor-saturable reactor are tuned to series resonance at the cornmutating frequency. For power generating loads, a coasting-feedback triac, diac, or dv/dt fired SCR is connected in parallel with the load and a bidirectional load current carrying device is used to return power to the supply and thereby provide a second mode of operation for the circuit. Inverters employ two cooperating pairs of circuits.

This is a division of application Ser. No. 334,690 filed Dec. 26, 1963 now Patent No. 3,360,712, and assigned to the same assignee as the present invention.

The present invention relates to a family of new and improved power circuits employing new controlled turn on conducting devices and a new and improved commutation scheme.

More particularly, the invention relates to a family of power circuits employing turn on, non-gate turn off solid state semiconductor control devices for power switching purposes, time ratio control of direct current electric power or for inversion of direct current electric power to alternating current electric power. By time ratio control of direct current electric power is meant the chop ping up of a direct current electric potential by controlling the on time of a turn on, turn off power switching device connected in circuit relationship with a load and the direct current electric potential. By inversion of direct current electric power to alternating current electric power is meant the switching of a load across alternate output terminals of a direct current electric supply by appropriately switching turn on, turn off power switching devices connecting the load in circuit relationship with the direct current electric supply.

.In recent years the turn on, turn off power switching devices employed in the above described types of power circuits for the most part have employed a solid state semiconductor device known as a silicon controlled rectifier (SCR). The SCR is a four layer PNPN junction device having a gating electrode which is capable of turning on current flow through the device with only a relatively small gating signal. The conventional SCR, however, is a non-gate turn off device in that once conduction through the device is initiated, the gate thereafter loses control over conduction through the device until it has been switched off by some external means. Such external means are generally referred to as commutation circuits,

3,487,234 Patented Dec. 30, 1969 and usually effect turning off of the SCR by reversal of the potential across the SCR. In addition to the SCR, recent advances in the semiconductor art have made available to industry new solid state semiconductor devices which are controlled turn on, non-gate turn off conducting devices, but which are bi-directional conducting devices. By oi-directional conducting device is meant the device is capable of conducting electric current in either direction through the device. One of these last mentioned devices, referred to as a Triac, is a gate controlled turn on NPNPN junction device which, similar to the SCR, is a non-gate turn off device that must be turned off by external commutation circuit means. While the preferred form of triac is a five-layer gate controlled device, it should be noted that four-layer PNPN and NPNP junction gate control triac devices are practical as well as other variations, but that in any event the triac character istics' mentioned above are common to all of them. The second newly available power device, referred to as a power diac is a two terminal, five layer NPNPN junction device which like the triac has bi-directional conducting characteristics. In contrast to the SCR and triac, however, the diac is not a gate turn on device, but must be turned on by the application of a relatively steep voltage pulse (high dv/dt) applied across its terminals. It should be noted that SCR and triacs may also be fired by the same high dv/dt technique. However, the diac is similar to the SCR and triac in that it too must be turned off by external circuit commutation means. The present invention provides new and improved power circuits employing solid state semi-conductor devices of the above general type as well as a new and improved commutation scheme for use with such devices. It is therefore a primary object of the present invention to provide an entire family of new and improved power circuits employing controlled turn on, non-gate turn off conducting devices.

Another object of the invention is to provide a new and improved commutation scheme for power circuits employing controlled turn on, non-gate turn off conducting devices which allows for a reduction in the size of components employed in the circuit for a given power rating, and hence is economical to manufacture.

A further object of the invention is the provision of such a new and improved commutation scheme which is economical and efi'icient in operation, and which provides reliable commutation that is independent of load from no load to full load operating conditions.

In practicing the invention, new and improved power circuits are provided using controlled turn on, non-gate turn off solid state semi-conductor devices. These new and improved power circuits include in combination a load current carrying turn on, non-gate turn off controlled conducting device and a load efiectively coupled in series circuit relationship across a pair of power supply terminals that in turn are adapted to be connected across a source of electric potential. Turn on gating and firing circuit means are provided for controlling the turn on of the controlled conducting devices, and commutation circuit means are provided for cornmutating off the devices at desired intervals. The commutation circuit means preferably comprises a linear inductance and a saturable reactor effectively connected in parallel circuit relationship, the saturated inductance of the saturable reactor being substantially less than the inductance of the linear inductance. A cornmutating capacitor is connected between one juncture of the parallel connected linear inductance and saturable reactor and one terminal of the load current carrying turn on, non-gate turn off controlled conducting device. The cornmutating capacitor and the linear inductance are tuned to series resonance at a. substantially lower frequency than a desired commutating frequency, and the commutating capacitor and the saturated inductance of the saturable reactor are tuned to series resonance at the desired commutating frequency. Means are provided including an auxiliary controlled turn on conducting device which is operatively connected between the remaining juncture of the parallel connected linear inductance and saturable reactor and one of the terminals of the load current carrying turn on, non-gate turn off controlled conducting device for circulating the charge on the commutating capacitor through a closed series circuit loop including the linear inductance and thereby reverse the polarity of the charge on the commutating capacitor. Means are also provided which include the saturable reactor for coupling the reverse polarity charge on the commutating capacitor across the load current carrying turn on, non-gate turn olf controlled conducting device to commutate it off.

Other objects, features and many attendant advantages of this invention will be appreciated more readily as the same becomes better understood by reference to the following detailed description, when considered in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIGURE 1 is a detailed circuit diagram of a new and improved time ratio control power circuit employing a new and improved commutation scheme made possible by the present invention;

FIGURE 2 is an equivalent circuit representation illustrating the time-ratio control principle together with a series of curves depicting the form of variable voltage direct current electric energy derived from time ratio control power circuits;

FIGURE 3 is an equivalent circuit diagram of a timeratio control circuit illustrating the effect of a coasting rectifier and filter inductance added to the equivalent circuit of FIGURE 2;

FIGURE 4 is a detailed circuit diagram of a new and improved time ratio control circuit similar to that in FIGURE 1, but modified to provide improved features of operation, and employing conventional gate controlled silicon control rectifier solid state devices;

FIGURE 5 is a series of voltage versus time characteristic curves illustrating the operation of the circuit shown in FIGURE 4;

FIGURE 6 is a detailed circuit diagram of a modification of the circuit of FIGURE 4, and illustrates a means for obtaining independent charging of the commutating capacitor employed in that circuit;

FIGURE 7 is a detailed circuit diagram of a modification of the circuit shown in FIGURE 6, and illustrates a means for obtaining charging of the commutating capacitor to a value dependent upon the value of the load current;

FIGURE 8 is a detailed circuit diagram of a suitable gating circuit for use with the time ratio control circuit showing in FIGURES l, 4, 6, and 7;

FIGURE 9 is a detailed circuit diagram of a modification of the gating circuit shown in FIGURE 8 to provide independent control over the commutation operation as well as independent control of turn on of the load current;

FIGURE 10 is a detailed circuit diagram of a modification of the circuit shown in FIGURE 6 which employs a triac in place of the auxiliary silicon control rectifier and coupling diode;

FIGURE 11 is a detailed circuit diagram of an all triac version of the circuit shown in FIGURE 10;

FIGURE 12 is a detailed circuit diagram of a new and improved time-ratio control circuit employing a a'v/dt fired SCR and the new and improved commutation scheme comprising a part of the present invention;

FIGURE 13 is a modification of the circuit shown in FIGURE 12 which uses a bi-directional conducting .4 diac in place of the dv/dt fired SCR, and a bi-directional conducting triac in place of the auxiliary SCR and coupling diode, and in addition illustrates a different form of firing circuit means for turning on a diac or dv/dt fired S'CR;

FIGURE 14 is a detailed circuit diagram of still a different form of firing circuit means for turning on a diac which uses common circuit elements to turn on the diac to conduct current in either one of two opposite directions;

FIGURE 15 is a modification of the circuit shown in FIGURE 14 which provides independent control of the turn on of the bi-directional conducting diac in either direction;

FIGURE 16 is a detailed circuit diagram of a new and improved time ratio control power circuit incorporating many of the features of the circuit shown in FIGURE 13, with the exception that a bi-directional conducting diac is substituted for the coasting rectifier of FIGURE 13 FIGURE 17 is a modification of the time ratio control power circuit shown in FIGURE 13 with the exception that a bi-directional conducting triac is substituted for the coasting rectifier of FIGURE 13;

FIGURE 18 is a modification of the time ratio control power circuit shown in FIGURE 16 with the exception that a convential gate fired SCR is substituted for the load current carrying diac of FIGURE 16;

FIGURE 19 is a modification of the time ratio control power circuit shown in FIGURE 11 with the exception that a bi-directional conducting diac is substituted for the coasting and feedback triac of FIGURE 11;

FIGURE 20 is a modification of the time ratio control power circuit shown in FIGURE 11 with the exception that a dv/dt fired SCR and feedback diode is substituted for the load current carrying triac of FIGURE 11, and also discloses a new form of firing circuit for diacs and dv/dt fired SCRs;

FIGURE 21 is a modification of the time ratio control power circuit shown in FIGURE 16 with the exception that a dv/dt fired SCR and feedback diode are substituted for the load current carrying diac of FIGURE 16;

FIGURE 22 is a modification of the time ratio control power circuit shown in FIGURE 11 with the exception that a coasting diode is substituted for the coasting and feedback triac of FIGURE 11;

FIGURE 23 is a detailed circuit diagram of a new and improved inverter circuit employing the new and improved commutation scheme made possible by the present invention, and which uses conventional gate fired SCRs;

FIGURE 24 is a detailed circuit diagram of a new and improved triac version of the inverter circuit shown in FIGURE 14; and

FIGURE 25 is a detailed circuit diagram of a new and improved diac version of the inverter circuit shown in FIGURE 14.

The new and improved time ratio control power circuit illustrated in FIGURE 1 of the drawing is comprised by a gate turn on, non-gate turn off solid state silicon controlled rectifier device 11 and a load 12 effectively coupled in series circuit relationship across a pair of power supply terminals 13 and 14 which in turn are adapted to be connected across a source of electric potential. In the particular embodiment of the invention shown in FIGURE 1, the source of electric potential is a direct current power supply having its positive potential applied to the terminal 13 and its negative potential applied to the terminal 14. It should be noted that while the TRC circuits herein disclosed are drawn in connection with direct current power supplies, with very little modification these circuits could be used to chop out any desired portion of a half cycle of applied alternating current potential. A filter inductance 15 is connected in series circuit relationship intermediate the S'CR 11 and load 12 and a coasting diode 16 is connected in parallel circuit relationship with the filter inductance and load 12.

Commutation circuit means are provided for turning off the SCR 11 which comprises a linear inductance 17 and a saturable reactor 18 effectively connected in parallel circuit relationship. The saturable reactor 18 is designed in such a manner that its saturated inductance is substantially less than the inductance of the linear inductance 17. For best operation of the circuit in most applications, it is desirable that a blocking diode 20 be connected in series circuit relationship with the linear inductance 17, and that saturable reactor 18 be connected in parallel with the series connected linear inductance 17 and blocking diode 20. A commutating capacitor 19 is interconnected intermediate one juncture of the parallel connected linear inductance 17 and saturable reactor 18 and the terminal 13 so that in effect the capacitor is electrically connected to the positive terminal of the SCR 11. The commutator capacitor 19 and the linear inductance 17 form a series circuit which is turned to series resonance at a frequency which is substantially lower than the desired commutating frequency. The commutating capacitor 19 and the saturated inductance of saturable reactor 18 likewise will be tuned to series resonance but at the desired commutating frequency which is substantially higher than the resonant frequency of linear inductance 17 and the commutating capacitor 19. Means are provided which include an auxiliary turn on controlled conducting device which is operatively connected between the remaining juncture of the parallel connected linear inductance 17 and saturable reactor 18 and the above mentioned terminal 13 which in effect is connected to the positive terminal of the SCR 11. In the embodiment of the invention shown in FIGURE 1 the auxiliary turn on controlled conducting device constitutes a second gate turn on, non-gate turn off solid state silicon controlled rectifier device 21 which serves to circulate the charge on the commutating capacitor 19 through a closed series circuit loop which includes the linear inductance 17, the additional or auxiliary SCR 21 and of course the commutating capacitor 19. By means of this connection, the polarity of the charge on the commutating capacitor 19 is reversed. The power circuit of FIG- URE 1 is completed by means comprising a coupling diode 22 having its negative electrode connected to the negative electrode of the SCR 11 and its positive electrode connected to the remaining juncture of the parallel connected linear inductance 17 and saturable reactor 18. By reason of this connection, the reverse polarity charge on the commutating capacitor 19 will be coupled through the saturable reactor 18 upon that reactor saturating, and through the coupling diode 22 to apply a reverse polarity potential across the SCR 11 to thereby commutate it off. Properly phased gating on signals are applied to the gating electrodes of the SCRs 11 and 21 from a suitable gating signal control circuit such as that shown in FIGURE 8 of the drawings for gating on the SCRs in properly timed sequence as explained hereinafter.

In operation, if it is assumed that initially the load current carrying SCR 11 is in its nonconducting or blocking state, then the commutating capacitor 19 will be charged through the load 12, filter inductance 15, diode 22 and saturable reactor 18 to essentially the full potential of the direct current power supply connected across the terminals 13 and 14. This charge will be positive at the terminal 13 and negative at the juncture of the linear inductance 17 and saturable reactor 18. In the process of charging the commutating capacitor 19, the saturable reactor 18 is driven into negative saturation and the potential across it is positive at dot end of the reactor. The circuit thereafter remains in this condition until such time that the gating on signal is applied to the gating on electrode of the SCR 11. Upon this occurrance,

the diode 22 blocks, and the charge on the commutating capacitor 19 is retained. Current supplied through the conducting SCR 11 is supplied through the filter inductance 15 and load 12 back to the direct current power supply. The SCR 11 will be allowed to remain conducting for a time period dependent upon the amount of current to be supplied to the load 12, and then will be commutated off in the manner of a time ratio control power circuit.

The theory of operation of time ratio power control is best illustrated in FIGURE 2 of the drawings wherein an on-oif switch 23 is shown connected in series circuit relationship with a load resistor 24 across a direct current power supply. With the arrangement of FIGURE 2 there are two possible types of operation in order to supply variable amounts of power to the load resistor 24. These two types of operation are to leave the switch 23 closed for a fixed period of time, and vary the time that the switch is open. This type of operation is illustrated in curves 2A wherein curve 2A(1) illustrates a condition where the switch 23 is opened for only a short period of time compared to the time it is closed to provide an average voltage across the load resistor 24 equal to about of the supply voltage E of the direct current power supply. In FIGURE 2A(2) the condition is shown where the switch 23, is left open for a period of time equal to that during which it is closed. Under this condition of operation the voltage across the load will equal approximately 50 percent of the supply voltage E FIG- URE 2A(3) illustrates the condition where the switch 23 is left open for a period of time equal to twice that for which the switch is closed so that the load voltage appearing across the load resistor 24 will be equal to about 25% of the supply voltage E In any event, it can be appreciated that by varying the period of time during which the switch 23 is left open the amount of direct current potential applied across the load 24 is varied proportionally.

In the second type of operation possible with time ratio control power circuits, the amount of time that the SCR 11 in the circuit of FIGURE 1 is left in its turned off or blocking condition can be varied to vary the load current through the load 12 proportionally. This second type of operation of the circuit shown in FIGURE 2 is illustrated in FIGURE 2B of the drawings wherein the amount of time that the switch 23 is closed is varied. In FIGURE 2B(1) the condition where the switch 23 is closed for a much greater period of time than it is open, is illustrated to provided a load voltage E of approximately .75 B In FIGURE 2B(2) the time that switch 23 is closed equals the time that it is open to produce a load voltage B is equal to .5 E In FIGURE 2B(3) the condition where the. switch 23 is left closed for a period of time equals about /2 the time that switch 23 is open to provide a load voltage E equal to .25 E It can be appreciated, therefore, that by varying the period of time that the switch 23 is left closed, the amount of voltage supplied across the load resistor 24 can be varied proportionally in a similar fashion to that described above with respect to switch 23, by varying the period of time that the SCR 11 of the circuit shown in FIGURE 1 either is in a conducting condition or a nonconducting condition, the power supplied to the load 12 can be varied proportionally. It is a matter of adjustment of the phasing of the gating control signals supplied to the control gates of the SCR 11 and the SCR 21 which determines the amount of time that the SCR 11 is either conducting or not conducting. This of course in turn determines the power supplied to the load 12 in the manner described with relation to FIGURE 2. Whether the amount of time that the SCR 11 is in its blocking condition, is varied, or whether the amount of time that the SCR 11 is conducting, is varied to provide such proportionally controlled power to the load 12 usually depends upon the load in question, but in so far as the principles of commutation to be described hereinafter are concerned, it does not matter which type of operation is employed.

FIGURE 3 of the drawings better depicts the nature of the output signal or voltage E developed across the load resistor 12 by the circuit shown in FIGURE 1. In FIGURE 3, the SCR 11 is again depicted by the on-oif switch 23, and the voltage or current versus time curves for the various elements of this circuit are illustrated in FIGURE 3B. FIGURE 3B(1) illustrates the voltage versus time characteristic of the potential E appearing across the coasting diode 16. It is to be noted that the potential E is essentially a square wave potential whose period is determined by the timing of the switch 23. For the period of time that the switch 23 is closed, a load current I flows through the filter inductance 15 and load 12 back into the power supply. On the switch 23 being opened (which corresponds to the SCR 11 being commutated olt to its blocking or non-conducting condition), the energy trapped in the filter inductance 15 will try to produce a coasting current flow in a direction such that it will be positive at the dot end of the filter inductance. This energy which is directly coupled across the coasting diode 16, causes the diode 16 to be rendered conductive, and to circulate a coasting current substantially equal to the load current I through the load 12 and coasting diode 16 thereby discharging the filter inductance 15. As a consequence, the load voltage E and for that matter the load current I will appear substantially as shown in FIGURE 3B(2), of the drawings as an essentially steady state value lower than the source voltage E, by a factor determined upon the timing of the on-off switch 23. This load voltage can be calculated from the expression shown in FIGURE 3. This expression states that the load voltage B is equal to the time that switch 23 is closed divided by time that switch 23 is closed plus the time switch 23 is open all multiplied by the power supply voltage E The current I supplied through switch 23 to the filter inductance 11 is illustrated in FIGURE 3B(3) of the drawing, and is essentially a square wave potential having the same period as the voltage a It should be noted that upon the next succeeding cycle of operation when the switch 23 is closed, the filter inductance -15 will again be charged in a manner such that when it discharges upon switch 23 being opened, its potential is positive at the dot end so that the coasting rectifier 16 is again rendered conductive, and discharges the filter inductance through the load 12 to provide the essentially continuous steady state load voltage E shown in FIGURE 3B(2) Coming back now to FIGURE 1 of the drawings, it can be appreciated that the timing of SCR 11 being switched on and commutated off determines essentially the load voltage E supplied across the load 12 in the manner discussed in connection with FIGURE 3 of the drawings. In order to commutate oft the SCR 11, the new and improved commutation circuit means comprised by the elements 17 through 22 has been provided. The new and improved commutation circuit means operates in accordance with the techniques of some presently known commutation circuits but accomplishes the commutation operation in an equally eificient and reliable manner with smaller components. One of these known commutation techniques employs a commutating capacitor connected in series circuit relationship with a saturable reactor across the SCR to commutate off the SCR. Another of the known commutation techniques employs a commutating capacitor, a series connected linear inductance which is tuned to series resonance with the commutating capacitor at the desired commutating frequency, and an auxiliary SCR used to reverse the charge across the commutating capacitor and hence commutate oif the main load current carrying SCR. The manner of operation of each of these previously known commutation techniques will be briefly discussed in order to appreciate better the operation of 8 the new and improved commutation circuit means shown in FIGURE 1.

The first known commutation technique to be described will be that known as McMurray commutation, and employs a tuned commutating capacitor and linear inductance and auxiliary SCR. During this portion of the description of McMurray commutation, the saturable reactor 18 and blocking diode 20 should be ignored as if they were not present in the circuit, and it is assumed that the tuned L-C network resonates at the desired commutating frequency. Assuming these conditions then, and that the SCR 11 is initially in its off or blocking condition, the commutating capacitor 19 will be charged to essentially the full potential of the direct current power supply through the series circuit comprised by load 12, filter inductance 15, coupling diode 22 and linear inductance 17. Thereafter, upon the SCR 11 being rendered conductive, the potential of the point 25 (which is essentially the potential of the cathode of the SCR 11 and shall be termed E goes immediately to the potential of the positive terminal 13. Upon this occurrence the diode 22 is rendered blocking so that the charge on the commutating capacitor 19 which is negative at the dot side of the capacitor at this point in the operating cycle, is trapped. Thereafter, some precalculated number of microseconds prior to the time that it is desired to commutate off the load current carrying SCR 11, the auxiliary SCR 21 is turned on by the application of a gating signal to its gate. The charge on the commutating capacitor 19 then oscillates in a sinusoidal fashion through the series tuned linear inductance 17 and conducting auxiliary SCR 21 to reverse the polarity of charge on the commutating capacitor 19 so that it now becomes positive at the dot side of the capacitor. This action results in driving the positive electrode of the coupling diode 22 positive with respect to the point 25 so that it is rendered conductive, and in turn drives the point 25 to a positive potential essentially twice that of the supply voltage E This is due to the fact that the commutating capacitor 19 is in elfect operatively coupled to the point 25 in series with the supply potential E Since the potential across the commutating capacitor 19 is essentially equal to that of the supply voltage E less any losses occasioned by the oscillation through the tuned L-C network and auxiliary SCR, the point 25 tries to go positive with respect to terminal 13 due to the impedance of filter inductance 15, thereby resulting in a reversal of the polarity of the potential across SCR 11 and causing it to be rendered non-conductive. The period of time that this condition is maintained is determined by the time required for the charge on the commutating capacitor 19 to leak off through the filter inductance 15 and load 12, and is sufficiently long to return SCR 11 to its blocking condition. Concurrently with SCR 11 being returned to its blocking condition the auxiliary SCR 21 likewise will be turned off, and returned to its blocking condition so as to condition the circuit for a new cycle of operation.

Considering now the second known commutation scheme known as the Morgan Circuit (after the present inventor), this known scheme utilizes a saturable reactor 18, and commutating capacitor 19, while ignoring the linear inductance 17, the blocking diode 20, the auxiliary SCR 21, and coupling diode 22, and assumes that the no dot end of the saturable reactor 18 is connected directly to the point 25. With such a commutation circuit, and with the SCR 11 turned off, the commutating capacitor 19 would be charged to essentially the full supply voltage E through the circuit including load 12, filter inductance 15, and saturable reactor 18. In thus charging the commutating capacitor 19, the saturable reactor 18 would be driven into positive saturation so that the voltage across the saturated reactor is positive at the dot end. Thereafter, upon the SCR 11 being rendered conductive by its gating control circuit, the point 25 goes positive with respect to the dot end of the saturable reactor 18 so that saturable reactor 18 is driven out of positive saturation towards negative saturation. During the period of time that the saturable reactor 18 is unsaturated, it will hold off the potential of the commutating capacitor 19 so that the commutating capacitor 19 stays fully charged to essentially the full potential of the supply voltage E The period of time that the saturable reactor 18 remains unsaturated is a matter of design of the saturable reactor. Hence, in the Morgan commutation circuit under consideration, the sat urable reactor really performs a timing function for determining how long the SCR 11 remains conductive. Accordingly, the charge on the commutating capacitor 19 is maintained until such time that the saturable reactor 18 is driven into negative saturation so that the potential across the reactor 18 is negative at the dot end. The saturable reactor also is designed so that its saturated inductance and the commutating capacitor 19 are tuned to series resonance at the desired commutating frequency. Hence upon reaching negative saturation the charge on the commutating capacitor 19 is oscillated 180 in a sinusoidal manner through the inductance of the saturated reactor back through the conducting SCR 11 to reverse the polarity of the charge on the commutating capacitor 19 so that it becomes positive at the dot side. The effect is to again reverse the polarity of the potential across the saturable reactor 18 so that saturable reactor 18 again is driven out of negative saturation back towards positive saturation. Hence for the period of time required for the saturable reactor 18 to again reach positive saturation the reverse polarity charge on the commutating capacitor 19 is maintained, and the SCR 11 continues to conduct. Upon the commutating saturable reactor 18 reaching positive saturation so that the potential across it is again positive at the dot end, the impedance of the saturable reactor essentially drops to zero. The reverse polarity charge on commutating capacitor 19 then is effectively coupled across the SCR 11 in the manner previously described in connection with the tuned linear inductance commutating capacitor auxiliary SCR type of McMurray commutation described above. This condition will be maintained for the period of time required for the reverse polarity charge to leak out through the saturated reactor 18, filter inductance 15, and load 12, and is a sufficient period of time to return the SCR 11 to its blocking condition. Upon this occurrence, commutating capacitor 19 will then again be recharged to the potential of the direct current power supply, and the circuit is conditioned for a new cycle of operation. For a more detailed description of the Morgan Circuit commutation scheme reference is made to US. Patent No. 3,019,355 entitled Magnetic Silicon Controlled Rectifier Power Amplifier," R. E. Morgan, inventor, issued Jan. 30, 1962.

Consider now the operation of the new and improved time ratio control power circuit shown in FIGURE 1 of the drawings, and assume that the load current carrying SCR 11 and auxiliary SCR 21 are turned off. With the circuit in this condition, the commutating capacitor 19 will be charged through the series circuit comprised by load 12, filter inductance 15, coupling diode 22, and saturable reactor 18 to essentially the full potential of the direct current power supply B In this charging operation the saturable reactor 18 will be driven into positive saturation so that the potential across the reactor 18 is positive at the dot end. Thereafter, upon the load current carrying SCR 11 being gated on by its gating control circuit, load current will be supplied to load 12 through filter inductance 15. Upon the SCR 11 being rendered conductive, the point 25 goes to essentially the potential of the positive bus bar 13 causing diode 22 to block, and thereby retaining the charge on the commutating capacitor 19. The circuit remains in this condition for the period of time that the SCR 11 is allowed to conduct as determined by the time ratio control principles described in connection with FIGURES 2 and 3.

Just prior to the time that the SCR 11 is to be commutated 01f, the auxiliary SCR 21 is turned on. Upon the auxiliary SCR 21 being turned on, the charge on commutating capacitor 19 is oscillated in a sinusoidal fashion through the blocking diode 20 (if present in the circuit), series tuned linear inductance 17, and conducting auxiliary SCR 21 so as to reverse the polarity of the charge across the commutating capacitor 19. This results in reversing the charge on commutating capacitor 19 and makes it positive at its dot side with respect to the terminal 13. With the circuit in this condition, the blocking diode 20 will block thereby preventing dissipation of the charge on commutating capacitor 19 through the linear inductance 17. In addition to this function, blocking diode 20 prevents ringing oscillations between linear inductance 17 and saturable reactor 18. It should be noted that if the blocking diode 20 is not used, and this is possible for some applications, some leakage of the reversed polarity charge on capacitor 19 will take place through linear inductance 17. However linear inductance 17 is so much larger than the saturable reactor 18 that this leakage will not seriously atfect operation of the circuit in an adverse manner. Use of the blocking diode 20 is preferred, however.

Concurrently with the reversal of the polarity of the charge on the commutating capacitor 19, the current flowing through the auxiliary SCR 21 splits and part of it drives the saturable reactor 18 out of positive saturation toward negative saturation while the bulk of it reverses the polarity of the charge on capacitor 19. During the interval of time that the saturable reactor 18 is unsaturated while it goes from positive saturation toward negative saturation, the charge on the commutating capacitor 19 will be held otf, and the SCR 11 as well as the auxiliary SCR 21 continue to conduct. Sometime during this interval, however, the polarity of the charge on capacitor 19 will have been reversed by the lower impedance path through inductance 17. Upon this reversal of potential the saturable reactor 18 will be driven back toward positive saturation. As a consequence, the saturable reactor 18 will remain unsaturated for an interval of time equal to that during which it was being driven toward negative saturation, and then again will be driven into positive saturation. Upon saturable reactor 18 again reaching positive saturation the reverse polarity charge on the commutating capacitor 19 will be coupled through the essentially zero impedance of the saturated reactor 18 and coupling diode 22 to drive the point 25, and therefore the potential a to a positive potential substantially equal to twice the value of the supply voltage terminal 13. This results in reversing the polarity of the potential across both the SCRs 11 and 21, and causes them to be turned off. This condition is maintained for the period of time required for the charge on the commutating capacitor 19 to leak off through the saturated reactor 18, filter inductance 15, and load 12. This period of time is, of course designed to allow a sufiicient period of time to assure the SCRs 11 and 21 return to their blocking condition thereby conditioning the circuit for a new cycle of operations.

It should be noted that during most of the commutation period described above, load current required by the load and filter inductance 15 is supplied through the load current carrying SCR 11 and need not be supplied through the auxiliary SCR 21 which serves only to reverse the polarity of the charge on the commutating capacitor 19. Since this reversal of charge occurs at a much lower frequency than the commutating frequency, and hence extends over a greater time period than the commutation interval, the auxiliary SCR 21 can be much smaller than that required for the tuned L-C, auxiliary SCR commutation scheme alone. Similarly, the size of the saturable reactor 18 can be greatly reduced over the size of a saturable reactor required if a commutating capacitor and saturating reactor alone were used for commutation of the load current carrying SCR 11. As a consequence,

a very considerable saving in the cost of the components of the commutation circuit is accomplished by reason of the new circuit configuration. In addition to these features, the circuit is highly eflicient in operation since the commutating energy is not dissipated but is recirculated, and then supplied to the load. As a consequence, the circuit is efficient and economical in operation, and provides reliable commutation.

FIGURE 4 of the drawings illustrates a modification of the circuit shown in FIGURE 1 of the drawings, and is preferred for applications where the load 12 being supplied is inductive in nature. The circuit of FIGURE 4 differs from the circuit of FIGURE 1 by the addition of a feed back dode 26 connected across the load current carrying SCR 11 in a verse polarity sense. By the inclusion of the feed back diode 26, when the potential ta at the point 25 tries to go positive with respect to the terminal 13, feed back diode 26 conducts, and clamps the potential of the point 25 to the potential of the terminal 13. As a result, current will be supplied back to the direct current power supply through the feed back diode 26, and, hence, it is sometimes referred to as a pump back diode. Because the potential at the point 25 therefore is clamped to the potential of the terminal 13 by feed back diode 26, the mode of commutation of SCRs 11 and 21 in the FIGURE 4 is somewhat different than the circuit of FIGURE 1. In the circuit arrangement of FIGURE 4, a reverse bias is developed across the SCRs 11 and 21 by reason of the forward voltage drop across the feed back diode 26. This voltage drop may be no more than 1 volt but it is sufficient to reverse the polarity of the potential across the SCRs 11 and 21, and causes them to turn off and return to their blocking condition.

FIGURE of the drawing is a series of potential versus time and current versus time characteristic curves illustrating the nature of the potentials and currents at different points in the power circuit of FIGURE 4. FIGURE 5A illustrates the potential e at point 25, and shows how it is clamped at essentially the power supply potential by feed back diode 26 during the commutation off of SCR 11 during the interval c-d of FIGURE 5B. FIGURE 5B illustrates the potential across the commutating capacitor 19 where it is shown that just prior to the time that the auxiliary SCR 21 is turned on, the potential E across the commutating capacitor 19 is negative during the time interval before point a. The time at which the auxiliary SCR 21 is turned on, corresponds to point a in FIGURE 5B and 5C, and the charge on commutating capacitor 19 then is oscillated 180 through inductance 17 over the time period abc to reverse the polarity of the charge on commutating capacitor 19. Thereafter, over the time period cde the reverse polarity charge on capacitor 19 drives saturable reactor 18 back into positive saturation, and turns off the SCRs 11 and 21. Turn off of SCRs 11 and 21 is essentially accomplished during the time interval cd, and during the interval of time de, the commutating capacitor 19 is recharged to the value of the supply potential as shrown in FIGURES 5B and SF. FIGURE SE of the drawings illustrates the current i flowing in the tuned L-C circuit comprised by commutating capacitor 19 and linear inductance 17 during the reversal of the charge on commutating capacitor 19. FIGURE SF of the drawings illustrates the commutation current t flowing out of the commutating capacitor 19 during the interval cdc, reaching a maximum at time d when the saturable reactor reaches saturation, and provides a pulse of current extending over a 20 microseond period which is the required time for most SCRs to assume a blocking condition. In curve 5D, the voltage across saturable reactor e is shown, and it can be seen that the energy under the portion of the curve ab which is required in driving saturable reactor 18 out of positive saturation towards negative saturation, equals the energy under the portion of the curve be required to drive the unsaturated reactor back into positive saturation.

FIGURE 6 of the drawing illustrates a modification of the circuit shown in FIGURE 4, and provides a means for assuring that the commutating capacitor 19 will be properly charged under all conditions of loading. Such assurance may be required because of the characteristics of a particular load 12 draws only very low load currents. This occurs where the load current may vary from no load to about 10% of full load. In order to insure that the commutating capacitor 19 will be properly charged for each commutating interval under such operating conditions, a charging inductance 27 and a charging diode 28 are connected in series circuit relationship between the negative power supply terminal 14 and the juncture of the commutating capacitor 19 with saturable reactor 18 and blocking diode 20. By this arrangement, the charging inductance 27 provides the energy to complete the recharging of the commutating capacitor 19 at the end of each commutating operation. During the commutating interval, the charging inductance 27 (which incidentally is considerably larger than the tuned inductance 17) stores energy equal approximately to /2 L .(L while the voltage across the inductor 27 is positive at the dot end. This occurs after the first reversal of the charge on commutating capacitor 19 and the dot side of capacitor 19 is positive. At the end of the commutation interval, the charging diode 28 blocks, and inductor 27 discharges into the commutating capacitor to charge it to a value greater than the supply voltage E With the charge on the commutating capacitor being greater than the supply potential, and the dot side of the commutating capacitor being negative, the diode 22 will stay blocked, and the charge on the commutating capacitor will be maintained. The inductor 27 is designed to discharge an amount of energy during the commutation interval which is equal to the losses in the commutating circuit for one commutating period. The cost in size, weight, and economy of operation made possible by the additional charging inductance 27 and charging diode 28 are considered small for the improved performance obtained at no load to 10 percent of full load operation conditions.

FIGURE 7 of the drawings illustrates a modification of the circuit shown in FIGURE 6 wherein the charging inductance 27 is inductively coupled to a primary winding 29 that may comprise a single turn or two connected in the coasting diode circuit in series circuit relationship with the coasting diode 16, filter inductance 15, and load 12. As a consequence of the connection, load current will be circulated through the primary winding 29 which will facilitate charging the inductance 27, and hence, commutating capacitor 19, to a value dependent upon the value of the load current I By this means, it is always assured that the commutating capacitor 19 will be charged to a value sufficient to assure commutation of the load current through SCR 11. If the load current I is equal to zero, or is of a very low value, essentially no extra energy will be transformed to the commutating capacitor 19, and hence, the operation of the commutating circuit will be the same as that described in connection with FIGURE 6. As the load current 1;, varies from a value equal to or nearly zero to full load, the charge on the commutating capacitor varies accordingly, and assures proper commutation.

FIGURE 8 of the drawings illustrates the construction of a gating circuit suitable for use with the new and improved power circuits shown in FIGURES 1, 4, 6 and 7. In FIGURE 8, a load current carrying silicon controlled rectifier device 11 is shown as having its gate electrode connected to the secondary winding of a pulse transformer 31. The primary winding of pulse transformer 31 is connected between one base of a unijunction transistor 32 and the negative terminal 14 of the direct current power supply. The remaining base of the unijunction transistor 32 is connected through a voltage limiting resistor 33 to the positive terminal of the direct current power supply. The emitter electrode of the unijunction transistor 32 is connected to the junction of a resistor 34 and capacitor 35 connected in series circuit relationship between the negative terminal 14 and the collector electrode of PNP transistor 36. The transistor 36 has its emitter electrode connected directly to the positive terminal 13, and its base electrode is connected to a source of control voltage for controlling the phasing of the time of firing of the load current carrying SCR 11.

In order to control the time of firing of an auxiliary SCR 21 at a fixed phase relationship with respect to the time of firing of the load current carrying SCR 11, the coasting rectifier 16 has its cathode connected to the cathode of a blocking diode 37. The coupling diode 37 in turn has its anode connected to the juncture of a resistor 38 and capacitor 39 connected in series circuit relationship across the terminals 13 and 14. The juncture of the resistor 38 and capacitor 39 is also connected to the emitter electrode of a unijunction transistor 41 which has one base connected through a resistor 42 to the positive terminal 13, and the remaining base connected through the primary winding of a pulse transformer 43 to the negative terminal 14. The secondary winding of the pulse transformer 43 is connected to the gate electrode of the auxiliary SCR 21.

By reason of the above-described arrangement, and the nature of the unijunction transistors 32 and 41, which are avalanche devices in that they are rendered fully conducting upon the base to emitter voltage of the device reaching a predetermined level, gating pulses will be produced in the primary windings of the pulse transformers 31 and 43 in the following manner. The control voltage applied to the base electrode of the PNP transistor 36 causes this transistor to vary the value of the resistance or resistance-capacitance network comprised by resistor-capacitor 34 and 35. This results in varying the rate at which the capacitor 35 is charged to a value sutficient to trigger on the unijunction transistor 32. Upon the unijunction transistor 32 being triggered on, a gating pulse will be produced in secondary winding of the pulse transformer 31 which turns on the load current carrying SCR 11. Upon the load current carrying SCR 11 being turned on, the point 25 is driven to the positive potential of terminal 13 so that the blocking diode 37 is rendered blocking. Upon the diode 37 being blocked the capacitor 39 will be charged up through the resistor 38 towards the potential of terminal 13 at a rate determined by the time constant of the resistor 38 and capacitor 39. This charging rate can be designed to provide a sufiicient potential across the capacitor 39 at a predetermined time interval after load current carrying SCR 11 is turned on to cause the unijunction transistor 41 to be turned on. This results in producing a gating pulse in the secondary winding of the pulse transformer 43 to thereby turn on the auxiliary SCR 21 at the desired fixed interval of time after load current carrying SCR 11 was turned on.

FIGURE 9 of the drawings illustrates a variation of the circuit shown in FIGURE 8 wherein independent control is provided over the firing of the auxiliary SCR. This independent control of the firing of auxiliary SCR 21 is achieved by the substitution of an additional PNP transistor 44 paralleled by a resistor 45, and connected in series circuit relationship with a resistor 46 in place of the fixed resistor 42 shown in FIGURE 7. By reason of this modification, variation of the conductance of the transistor 44 will operate to vary the resistance of the resistance network comprised by transistor 44, resistor 45, and resistor 46, to thereby vary the charging rate of the capacitor 39. This in turn varies the time, at which the unijunction transistor 41 is turned full on resulting in gating on the auxiliary SCR 21 with respect to the turn on time of the load current carrying SCR 11. If desired,

other forms of suitable firing circuits for the power circuit arrangements herein described may be used, such as those disclosed in chapter 9 entitled Inverter and Chopper circuits of the Silicon Controlled Rectifier Manual published by the General Electric Company, Rectifier Components Department, copyrighted in 1961.

FIGURE 10 of the drawings illustrates a modification of the time ratio control power circuit shown in FIG- URE 6 wherein the auxiliary SCR 21 and coupling diode 22 of the FIGURE 6 circuit arrangement are replaced by a single gate turn on, non-gate turn off solid state triac bi-directional conducting device 51. The triac 51 is a gate turn on, non-gate turn off, bi-directional conducting device which has been newly introduced to the electrical industry by the Rectifier Components Department of the General Electric Company, Auburn, NY. Similar to the SCR, the triac may be switched from a high impedance blocking state to a low impedance conducting state when a low voltage gate signal is applied between the gate terminal and one of the load terminals. Also, like the SCR, once the triac is switched to the low impedance conducting state, the gate electrode loses control and current flow through the device must be interrupted by some external means while the gate signal is removed in order to return the triac to its high impedance blocking state. A further characteristic of the triac 51 is that once it is gated on, it will conduct current through the device in both directions depending upon the polarity of the potential across the device. For a more detailed description of the triac gate turn on, non-gate turn off solid state semiconductor device, reference is made to the U.S. application Ser. No. 331,77 6 entitled Semiconductor Switch, by F. W. Gutzwiller, inventor, filed Dec. 20, 1963, and assigned to the General Electric Company.

Gating circuit means are provided for switching triac 51 to its low impedance conducting state, and includes first gating circuit means to gate on the triac device 51 to cause it to conduct current when the potential of point 25 is positive with respect to the dot side of commutating capacitor 19. This first gating circuit means includes a limiting resistor 47 and the secondary winding of a gating pulse transformer 48 connected in series circuit re lationship between the control gate of triac 51 and point 25 which is at the same potential as the negative terminal of load current carrying SCR 11. In order to turn on triac 51 when the potential of the dot side of commutating capacitor 19 is positive with respect to point 25, the gating circuit means includes a second gating circuit comprised by a coupling diode 49 and limiting resistor 50 connected in series circuit relationship between the control gate of triac 51 and the remaining juncture of the parallel connected linear inductance and saturable reactor opposite that to which the commutating capacitor 19 is connected. To operate properly, the anode of diode 49 must be effectively connected to the last mentioned remaining juncture, and the cathode of diode 49 effectively connected to the control gate of triac 51.

In operation the circuit of FIGURE 10 will function in much the same manner as the circuit of FIGURE 6. During the interval when the load current carrying SCR 11 is turned off, the commutating capacitor 19 is charged to a potential greater than the direct current power supply by inductance 27 in the previously described manner. Thereafter, upon the load current carrying SCR 11 being turned on, the triac 51 (which at this point is in its blocking condition) prevents the charge on capacitor 19 from being dissipated through load 12. At some predetermined time interval prior to the time for turning off the load current carrying SCR 11, a gating on signal is applied to the triac 51 by gating lpulse transformer 48. Upon this occurrence, the charge on commutating capacitor 19 is oscillated through the linear inductance 17, blocking diode 20, triac 51, and load current carrying SCR 11 to reverse the polarity of the charge on the commutating capacitor. Upon the polarity of the charge on 

